Before the invention of standard time zones, each city or region could have its own local time. This became increasingly awkward as railways and telecommunications improved.
Originally, time zones based their time on Greenwich Mean Time (GMT). Mean solar time is defined by the rotation of the Earth, which is not constant in rate. Starting January 1, 1972, a new system was used, Coordinated Universal Time (UTC), which used a fixed rate and added leap seconds when necessary to compensate for variations in the rotation of the Earth.
In theory, there are 24 time zones, making each a constant 15 degrees of longitude apart. A time zone varies in width from zero miles at both poles to over 1000 miles at the equator.
In reality, there are about 40 time zones, and the border between time zones is irregular, following political or geographical boundaries. The island of Newfoundland, India, and parts of Australia use half-hour deviations from standard time, and some nations use quarter-hour deviations.
Canada's Sir Sandford Fleming first proposed time zones for the entire world in 1876. Most major countries had adopted hourly time zones by 1929.